Abstract:

The invention produces a solid electrolytic capacitor using a solid
electrolytic capacitor element by a method comprising forming a
dielectric layer on the surface of an electric conductor, forming a
semiconductor layer containing electrically conducting polymer on the
dielectric layer and forming an electrode layer thereon, wherein the
dielectric layer is formed by chemical formation in an electrolytic
solution containing a dopant.

Claims:

1. A method for producing a solid electrolytic capacitor element,
comprising forming a dielectric layer on the surface of an electric
conductor, forming a semiconductor layer containing electrically
conducting polymer on the dielectric layer and forming an electrode layer
thereon, wherein the dielectric layer is formed by chemical formation in
an electrolytic solution containing a dopant, and wherein the dopant is
an electron-donating compound which gives an electrically conducting
polymer having an electric conductivity of 10.sup.1 to 10.sup.3
Scm-1 when doped at the time of electrolytic polymerization.

2. The method for producing a solid electrolytic capacitor element as
claimed in claim 1, wherein the dopant is the same as the dopant
contained in the electrically conducting polymer constituting the
semi-conductor layer.

3. (canceled)

4. The method for producing a solid electrolytic capacitor element as
claimed in claim 1, wherein the dopant is at least one member selected
from compounds having a sulfonic acid group.

5. The method for producing a solid electrolytic capacitor element as
claimed in claim 4, wherein the dopant is at least one member selected
from quinone sulfonic acids which may be substituted.

6. The method for producing a solid electrolytic capacitor element as
claimed in claim 1, wherein the dopant is at least one member selected
from boron compounds in which a carboxylic acid is coordinated to a boron
atom.

7. The method for producing a solid electrolytic capacitor element as
claimed in claim 1, wherein chemical formation is further performed again
in the electrolytic solution described above.

8. The method for producing a solid electrolytic capacitor element as
claimed in claim 1, wherein the electric conductor is a metal or alloy
mainly comprising at least one member selected from a group consisting of
tantalum, niobium, titanium and aluminum; a niobium oxide; or a mixture
of at least two of the members selected from these metals, alloy and
niobium oxide.

9. The method for producing a solid electrolytic capacitor element as
claimed in claim 1, wherein the semiconductor layer is at least one layer
selected from semiconductors mainly comprising an electrically conducting
polymer obtained by doping a dopant in a polymer containing a repeating
unit represented by the following formula (1) or (2):wherein R1 to
R4 each independently represents a hydrogen atom, an alkyl group
having from 1 to 6 carbon atoms or an alkoxy group having from 1 to 6
carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen
atom, R5 is present only when X is a nitrogen atom, and represents a
hydrogen atom or an alkyl group having from 1 to 6 carbon atoms, and each
pair of R1 with R2 and R3 with R4 may combine to form
a cyclic structure.

10. The method for producing a solid electrolytic capacitor element as
claimed in claim 9, wherein the polymer containing a repeating unit
represented by formula (1) is a polymer containing, as a repeating unit,
a structural unit represented by the following formula (3):wherein
R6 and R7 each independently represents a hydrogen atom, a
linear or branched, saturated or unsaturated alkyl group having from 1 to
6 carbon atoms, or a substituent forming at least one or more 5-, 6- or
7-membered saturated hydrocarbon cyclic structure containing two oxygen
atoms when the alkyl groups are combined with each other at an arbitrary
position, and the cyclic structure includes a structure having a vinylene
bond which may be substituted, and a phenylene structure which may be
substituted.

11. The method for producing a solid electrolytic capacitor element as
claimed in claim 9, wherein the electrically conducting polymer is
selected from a group consisting of polyaniline, polyoxyphenylene,
polyphenylene sulfide, polythiophene, polyfuran, polypyrrole,
polymethylpyrrole, and a substitution derivative and a copolymer thereof.

12. The method for producing a solid electrolytic capacitor element as
claimed in claim 10, wherein the electrically conducting polymer is
poly(3,4-ethylene-dioxythiophene).

13. The method for producing a solid electrolytic capacitor element as
claimed in claim 9, wherein the electric conductivity of the
semiconductor is from 10.sup.-2 to 10.sup.3 Scm.sup.-1.

14. A solid electrolytic capacitor element obtained by the production
method described in claim 1.

16. An electronic circuit using the solid electrolytic capacitor described
in claim 15.

17. An electronic device using the solid electrolytic capacitor described
in claim 15.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a production method of a solid
electrolytic capacitor element with good reliability.

BACKGROUND ART

[0002]As for a capacitor having high capacitance and low ESR (equivalent
series resistance) used in various electronic devices, an aluminum solid
electrolytic capacitor and a tantalum solid electrolytic capacitor are
known.

[0003]The solid electrolytic capacitor is produced by sealing a solid
electrolytic capacitor element in which an aluminum foil having fine
pores in the surface layer or a tantalum powder sintered body having fine
pores in the inside is used as one electrode (electric conductor) and
which comprises a dielectric layer formed on the surface layer of the
electrode, the other electrode (usually a semiconductor layer) provided
on the dielectric layer, and an electrode layer stacked on the other
electrode. In comparison among capacitors using electric conductors
having the same volume, the smaller the size of the fine pores of the
conductor and the larger the number of the pores, the larger the surface
area of the conductor inside and the larger the capacitance of the
capacitor produced from the electric conductor can be.

[0004]The dielectric layer is formed by an electrochemical method called
chemical formation. An example of the forming process is a method where
an electrically conducting layer is dipped in an electrolytic solution
containing a mineral acid (e.g., phosphoric acid, sulfuric acid) or a
salt thereof, or an organic acid (e.g., acetic acid, adipic acid, benzoic
acid) or a salt thereof dissolved therein and a predetermined voltage is
applied between the electric conductor serving as an anode and a cathode
separately provided in the electrolytic solution. A part of the
electrolyte used for the chemical formation is incorporated into the
dielectric layer.

[0005]JP-A-S50-100570 (the term "JP-A" as used herein means an "unexamined
published Japanese patent application"; patent Document 1, related
application; U.S. Pat. No. 3,864,219) describes chemical formation in an
electrolytic solution using quaternary ammonium salt. Also,
JP-A-S50-102861 (patent Document 2) describes chemical formation in an
electrolytic solution using boric acid or the like.

[0006]As for the semiconductor layer, an organic or inorganic compound is
used but in the light of heat resistance or low ESR property of the
produced capacitor, an electrically conducting polymer is used in
preference. The electrically conducting polymer is a polymer having a
high electric conductivity of 10-2 to 103 Scm-1. The high
electric conductivity is prepared by adding an electron-donating compound
called a dopant to a polymer having a planer conjugated double bond
(generally, insulating material or a polymer having a very low electric
conductivity). Specific examples of the method for forming an
electrically conducting polymer as the semiconductor layer include a
method of polymerizing a monomer capable of being polymerized to an
electrically conducting polymer in the fine pores of the electric
conductor by supplying thereto an appropriate oxidizing agent or an
electron in the presence of a dopant. The dopant is entrained upon
polymerization of the monomer and strong interaction with the polymer
having a conjugated double bond occurs, whereby an electrically
conducting polymer is obtained.

[0007]A solid electrolytic capacitor is required to have a high
reliability. One example of examining the reliability by acceleration is
a high heat load test. In the test, for example, solid electrolytic
capacitors are left standing at 105° C. for thousands of hours
while applying a rated voltage of the capacitor and those not whose
electric properties have not degraded are determined as acceptable.

[Patent Document 1]

[0008]Japanese Patent Application Laid-Open No. S50-100570

[Patent Document 2]

[0009]Japanese Patent Application Laid-Open No. S50-102861

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

[0010]Recent electronic devices are desired to be small in size and have
large capacitance. Solid electrolytic capacitors having large surface
area to meet such demands involve a problem that their electric
properties, especially dielectric tangent performance, are easy to
deteriorate under the high heat load test.

[0011]Accordingly, an object of the present invention is to provide a
high-capacitance solid electrolytic capacitor with high reliability.

Means for Solving the Problem

[0012]As a result of intensive investigations to solve the above-described
problem, the present inventors have found that when a dielectric layer of
a solid electrolytic capacitor using a semiconductor layer comprising at
least an electrically conducting polymer is formed by chemical formation
in an electrolytic solution containing a dopant, a solid electrolytic
capacitor exhibiting high reliability can be obtained. The present
invention has been accomplished based on this finding.

[0013]That is, the present invention provides a production method of a
solid electrolytic capacitor element, a solid electrolytic capacitor
produced by using the method, and uses thereof, as follows.

1. A method for producing a solid electrolytic capacitor element,
comprising forming a dielectric layer on the surface of an electric
conductor, forming a semiconductor layer containing electrically
conducting polymer on the dielectric layer and forming an electrode layer
thereon, wherein the dielectric layer is formed by chemical formation in
an electrolytic solution containing a dopant.2. The method for producing
a solid electrolytic capacitor element as described in 1 above, wherein
the dopant is the same as the dopant contained in the electrically
conducting polymer constituting the semiconductor layer.3. The method for
producing a solid electrolytic capacitor element as described in 1 above,
wherein the dopant is an electron-donating compound which gives an
electrically conducting polymer having an electric conductivity of
101 to 103 Scm-1 when doped at the time of electrolytic
polymerization.4. The method for producing a solid electrolytic capacitor
element as described in any one of 1 to 3 above, wherein the dopant is at
least one member selected from compounds having a sulfonic acid group.5.
The method for producing a solid electrolytic capacitor element as
described in 4 above, wherein the dopant is at least one member selected
from quinone sulfonic acids which may be substituted.6. The method for
producing a solid electrolytic capacitor element as described in any one
of 1 to 3 above, wherein the dopant is at least one member selected from
boron compounds in which a carboxylic acid is coordinated to a boron
atom.7. The method for producing a solid electrolytic capacitor element
as described in 1 or 2 above, wherein chemical formation is further
performed again in the electrolytic solution described above.8. The
method for producing a solid electrolytic capacitor element as described
in 1 above, wherein the electric conductor is a metal or alloy mainly
comprising at least one member selected from a group consisting of
tantalum, niobium, titanium and aluminum; a niobium oxide; or a mixture
of at least two of the members selected from these metals, alloy and
niobium oxide.9. The method for producing a solid electrolytic capacitor
element as described in 1 above, wherein the semiconductor layer is at
least one layer selected from semiconductors mainly comprising an
electrically conducting polymer obtained by doping a dopant in a polymer
containing a repeating unit represented by the following formula (1) or
(2):

wherein R1 to R4 each independently represents a hydrogen atom,
an alkyl group having from 1 to 6 carbon atoms or an alkoxy group having
from 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a
nitrogen atom, R5 is present only when X is a nitrogen atom, and
represents a hydrogen atom or an alkyl group having from 1 to 6 carbon
atoms, and each pair of R1 with R2 and R3 with R4 may
combine to form a cyclic structure.10. The method for producing a solid
electrolytic capacitor element as described in 9 above, wherein the
polymer containing a repeating unit represented by formula (I) is a
polymer containing, as a repeating unit, a structural unit represented by
the following formula (3):

wherein R6 and R7 each independently represents a hydrogen atom,
a linear or branched, saturated or unsaturated alkyl group having from 1
to 6 carbon atoms, or a substituent forming at least one or more 5-, 6-
or 7-membered saturated hydrocarbon cyclic structure containing two
oxygen atoms when the alkyl groups are combined with each other at an
arbitrary position, and the cyclic structure includes a structure having
a vinylene bond which may be substituted, and a phenylene structure which
may be substituted.11. The method for producing a solid electrolytic
capacitor element as described in 9 above, wherein the electrically
conducting polymer is selected from a group consisting of polyaniline,
polyoxyphenylene, polyphenylene sulfide, polythiophene, polyfuran,
polypyrrole, polymethylpyrrole, and a substitution derivative and a
copolymer thereof.12. The method for producing a solid electrolytic
capacitor element as described in 10 or 11 above, wherein the
electrically conducting polymer is poly(3,4-ethylene-dioxythiophene).13.
The method for producing a solid electrolytic capacitor element as
described in 9 above, wherein the electric conductivity of the
semiconductor is from 10-2 to 103 Scm-1.14. A solid
electrolytic capacitor element obtained by the production method
described in any one of 1 to 13 above.15. A solid electrolytic capacitor
obtained by sealing the solid electrolytic capacitor element described in
14 above.16. An electronic circuit using the solid electrolytic capacitor
described in 15 above.17. An electronic device using the solid
electrolytic capacitor described in 15 above.

EFFECTS OF THE INVENTION

[0014]The present invention provides a method for producing a solid
electrolytic capacitor element, wherein a dielectric layer is formed by
chemical formation in an electrolytic solution containing a dopant, and
also provides a solid electrolytic capacitor obtained by sealing a
capacitor element produced by the production method. According to the
present invention, a high-capacitance solid electrolytic capacitor with
high reliability can be produced.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015]The production method of a solid electrolytic capacitor element
according to the present invention and one embodiment of the solid
electrolytic capacitor produced by the method are described below.

[0016]Examples of the electric conductor for use in the present invention
include a metal or alloy mainly comprising at least one member selected
from tantalum, niobium, titanium and aluminum; a niobium oxide; and a
mixture of at least two members selected from these metals, alloy and
niobium oxide.

[0017]In the case of using a metal as the electric conductor, the metal
may be used after subjecting a part thereof to at least one treatment
selected from carbonization, phosphation, boronation, nitridation and
sulfidation.

[0018]The electric conductor is not particularly limited in its shape and
is used, for example, in the form of foil, plate or bar or in a form
created by molding or sintering after molding the powder of the electric
conductor. The electric conductor may also be foil-like or plate-like
metal prepared by attaching powdery electric conductor thereto and
sintering it. Furthermore, the electric conductor may be surface-treated
by etching or the like to produce fine pores. In a case where the
electric conductor is pulverized and molded or is pulverized and then
sintered after molded, fine pores can be produced in the inside after
molding or sintering by appropriately selecting the pressure at the time
of molding.

[0019]An outgoing lead may be connected directly to the electric
conductor. In a case where the electric conductor is pulverized and
molded or is pulverized and then sintered after molded, it is also
possible to mold a part of a separately prepared lead wire (or a lead
foil) together with the powder and use the part of the outgoing lead wire
(or lead foil) outside the molded portion as an outgoing lead for one
electrode of the solid electrolytic capacitor element. Or, a part of the
electric conductor, which part is left without semiconductor layer
formed, may be used as an anode. At the boundary between the anode part
and the semiconductor layer-forming part, an insulating resin may be
attached and cured like a belt so as to prevent the semiconductor layer
from crawling up.

[0020]Preferred examples of the electric conductor for use in the present
invention include an aluminum foil with the surface being etched, and a
sintered body having many fine pores in the inside, which is obtained by
molding and then sintering a powder such as tantalum powder, niobium
powder, alloy powder mainly comprising tantalum, alloy powder mainly
comprising niobium, and niobium monoxide powder.

[0021]In a case where a sintered body is to be prepared by molding a
powder and then sintering the compact, the produced sintered body can
have a large specific surface area per mass by using a powder having a
small particle diameter. In the present invention, it is preferred that
the CV value (value obtained by dividing a product of capacitance and
chemical formation voltage described later by mass) be 80,000 μFV/g or
more in the case of tantalum powder or 150,000 μFV/g or more in the
case of niobium powder or niobium monoxide powder and that the mass be 20
mg or more, preferably 50 mg or more, because the produced solid
electrolytic capacitor element can have a large capacitance with a small
volume.

[0022]Examples of the dielectric layer formed on the surface of the
electric conductor of the present invention include a dielectric layer
mainly comprising at least one member selected from metal oxides such as
Ta2O5, Al2O3, TiO2 and Nb2O5. It is
important that such a dielectric layers is formed by chemical formation
in an electrolytic solution containing a dopant in the present invention.

[0023]The dopant is a compound which can cause an effect to convert a
polymer compound an electric conductor when doped chemically or
electrochemically to the polymer compound having a conjugated double bond
at its main chain. For example, the dopant is an electron-donating
compound which gives an electrically conducting polymer having an
electric conductivity of 101 to 103 Scm-1 when pyrrole or
3,4-ethylenedioxythiophene is used as a representative monomer and the
dopant is doped simultaneously with electrolytic polymerization of the
monomer.

[0024]Preferred specific examples of the dopant include a compound having
a sulfonic acid group, and a boron compound in which a carboxylic acid is
coordinated to the boron atom. Representative examples of such a compound
include sulfonic acids having an aromatic ring or an alkyl substituted
aromatic ring, such as benzenesulfonic acid, toluenesulfonic acid,
naphthalenesulfonic acid and anthracenesulfonic acid; quinonesulfonic
acids such as benzoquinonesulfonic acid, naphthoquinonesulfonic acid and
anthraquinonesulfonic acid; sulfonic acids having an alkyl group, such as
butylsulfonic acid, hexylsulfonic acid and cyclohexylsulfonic acids;
various oligomer or polymer (polymerization degree: from 2 to 200)
sulfonic acids such as polyvinylsulfonic acid; and salts (e.g., ammonium
salt, alkali metal salt, alkaline earth metal salt and other metal salt)
of these sulfonic acids. These compounds may have various substituents
and may have a plurality of sulfonic acid groups. Examples thereof
include 2,6-naphthalenedisulfonic acid and 1,2-ethane disulfonic acid.
Examples of the boron compound include ammonium borodisalicylate, a
hydrate thereof and boro-1,2-carboxybenzene ammonium. As for the dopant,
two or more dopants may be used in combination. Among these dopants,
preferred are nonsurfactant-type dopants such as a quinonesulfonic acid
and a salt thereof, because the solid electrolytic capacitor produced by
forming a dielectric layer using the dopant has good reliability. In
unsubstituted quinonesulfonic acids cited in the above, quinonesulfonic
acids substituted with a lower alkyl group are also included in the
present invention.

[0025]Furthermore, it is preferable to use the same dopant as the dopant
contained in the electrically conducting polymer constituting the
semiconductor layer, that is, the dopant used for doping at the same time
as the polymerization by an electrolytic polymerization, because the
produced solid electrolytic capacitor exhibits a lower ESR value.

[0026]The dopant for use in the present invention is described as a
compound. When the compound is used as dopant, the compound is in a state
that a part of the compound is charged (δ-) or ionized (mostly
anion) and those compounds in such a state are also included in the scope
of constituent elements of the present invention (for example, in the
case of a benzenesulfonic acid, benzenesulfonate anion is also included).

[0027]The concentration of the dopant used therein is determined by taking
account of reliability of a produced solid electrolytic capacitor, but
usually a few tens of percent or less.

[0028]The electrolytic solution containing a dopant of the present
invention is a solution where at least one kind of the dopant described
above is dissolved or a part of the dopant is suspended in organic
solvent such as water and/or various alcohols, various esters and various
grimes. In a case where the electrolytic solution is an aqueous solution,
the electrolytic solution can be an aqueous solution for chemical
formation. As for the electrolyte in the chemical formation, at least one
kind of conventionally known electrolytes such as a mineral acid (e.g.,
phosphoric acid, sulfuric acid, boric acid) or a salt thereof, or an
organic acid (e.g., acetic acid, adipic acid, benzoic acid, nitrobenzoic
acid) or a salt thereof may be dissolved or may be suspended in part.

[0029]Furthermore, the electric conductor of the present invention may be
subjected to chemical formation in an electrolytic solution containing a
known electrolyte before and after formation of the dielectric layer in
an electrolytic solution containing a dopant of the present invention.
After each chemical formation, cleaning and drying process may be
provided so as to remove the electrolytic solution used for the chemical
formation.

[0030]This dielectric layer can be formed by dipping the electric
conductor in an electrolytic solution, and applying a voltage between the
electric conductor serving as the anode and a cathode plate separately
disposed in the electrolytic solution (this treatment is called "chemical
formation"). The conditions of chemical formation, such as chemical
formation temperature, chemical formation time and current density at
chemical formation, are determined by taking account of the type, mass
and size of the electric conductor, the capacitance and operating voltage
of the objective solid electrolytic capacitor element or the like. The
chemical formation temperature is usually from room temperature to
100° C., and the chemical formation time is usually from several
hours to several days.

[0031]It is assumed that, when the dielectric layer is formed by chemical
formation in an electrolytic solution containing a dopant in the present
invention, a trace amount of dopant is incorporated into the dielectric
layer. It can be presumed that dopant with one part being incorporated in
the inside of the dielectric layer and the other part being outside the
surface of the dielectric layer interacts with the electrically
conducting polymer constituting the semiconductor layer described later
to thereby play a role in linking firmly the dielectric layer with the
electrically conducting polymer, and thus the dopant prevents dielectric
tangent from deteriorating due to separation of the electrically
conducting polymer from the dielectric layer during the high heat load
test.

[0032]Meanwhile, the other electrode formed on the dielectric layer of the
present invention includes at least one organic semiconductor selected
from electrically conducting polymers described later. The organic
semiconductor contains an electrically conducting polymer an essential
component, and may further contain at least one compound selected from
other organic semiconductors and inorganic semiconductors as a layer or
as a mixture.

[0033]Specific examples of the organic semiconductor include an organic
semiconductor comprising benzopyrroline tetramer and chloranil, an
organic semiconductor mainly comprising tetrathiotetracene, an organic
semiconductor mainly comprising tetracyanoquinodimethane, and an organic
semiconductor mainly comprising an electrically conducting polymer
obtained by doping a dopant in a polymer containing a repeating unit
represented by the following formula (1) or (2):

wherein R1 to R4 each independently represents a hydrogen atom,
an alkyl group having from 1 to 6 carbon atoms or an alkoxy group having
from 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a
nitrogen atom, R5 is present only when X is a nitrogen atom, and
represents a hydrogen atom or an alkyl group having from 1 to 6 carbon
atoms, and each pair of R1 with R2 and R3 with R4 may
combine to form a cyclic structure.

[0034]In the present invention, the polymer containing a repeating unit
represented by formula (I) is preferably a polymer containing, as a
repeating unit, a structural unit represented by the following formula
(3):

wherein R6 and R7 each independently represents a hydrogen atom,
a linear or branched, saturated or unsaturated alkyl group having from 1
to 6 carbon atoms, or a substituent forming at least one 5-, 6- or
7-membered saturated hydrocarbon cyclic structure containing two oxygen
atoms when the alkyl groups are combined with each other at an arbitrary
position. The cyclic structure includes a structure having a vinylene
bond which may be substituted, and a phenylene structure which may be
substituted.

[0035]The electrically conducting polymer containing such a chemical
structure is being electrically charged and a dopant is doped therein.
The dopant is not particularly limited and a known dopant can be used.

[0036]Preferred examples of the dopant include compounds mentioned as
dopant examples which may be used in forming a dielectric layer by
chemical formation in an electrolytic solution containing the dopant. It
is true of the dopant here that two or more dopants may be used in
combination.

[0037]Examples of the polymer containing a repeating unit represented by
formula (1), (2) or (3) include polyaniline, polyoxyphenylene,
polyphenylene sulfide, polythiophene, polyfuran, polypyrrole,
polymethylpyrrole, and a substitution derivative and a copolymer thereof.
Among these, preferred are polypyrrole, polythiophene and a substitution
derivative thereof (e.g., poly(3,4-ethylenedioxythiophene)).

[0038]The above-described semiconductor layer is formed by a pure chemical
reaction (for example, a solution reaction, a vapor phase reaction, a
solid-liquid reaction or a combination thereof), an electrolytic
polymerization technique, or a combination of these methods. It is
preferred that the semi-conductor layer be produced by employing an
electrolytic polymerization technique at least once, in that the initial
ESR value can be low as compared with other methods, presumably that is
because no branching is generated in the electrically conducting polymer
chain or because thickness of the semiconductor layer on the outer
surface layer of the electric conductor can be uniform by this technique.

[0039]Specific examples of the inorganic semiconductor include at least
one compound selected from molybdenum dioxide, tungsten dioxide, lead
dioxide and manganese dioxide.

[0040]When the organic or inorganic semiconductor used has an electric
conductivity of 10-2 to 103 Scm-1, the solid electrolytic
capacitor produced can have a small ESR value and this is preferred.

[0041]In order to repair fine defects of the dielectric layer, generated
during the formation of the semiconductor layer, re-chemical formation
may be performed. Moreover, the operation of forming a semiconductor
layer and then re-chemical formation can be repeated multiple times, and
the conditions of each operation while repeating the operation may be
flexible. Usually, when the operation of a semiconductor layer formation
is stopped, the electric conductor is pulled up from a solution for a
semiconductor layer formation then washed and dried. The re-chemical
formation operation may be performed after repeating operations of
formation of semiconductor layer/stopping of the semiconductor layer
formation/washing/drying twice or more. Although the reason why is not
known exactly, mass of the semiconductor layer is increased in some cases
where operations of formation of semiconductor layer/stopping of the
semiconductor layer formation/washing/drying are repeated as compared
with cases where the operation of forming a semiconductor layer is
continuously performed, if the total time for forming a semiconductor
layer is the same.

[0042]The re-chemical formation may be performed in the same manner as the
above-described method of forming a dielectric layer by chemical
formation, or may be performed in a known electrolytic solution. However,
it is preferred to perform the re-chemical formation in the same
electrolytic solution as used in forming a dielectric layer of the
present invention, in that the ESR value of the produced solid
electrolytic capacitor can become low. The re-chemical formation voltage
is usually lower than the chemical formation voltage.

[0043]Furthermore, as a pretreatment before semiconductor layer formation,
minute protruding portions may be formed as small electrically defect
portions on the dielectric layer formed on the surface of an electric
conductor for the purpose of enhancing formation of a semiconductor layer
to be formed thereon.

[0044]In a case where formation of a semiconductor layer is divided into
two or more steps, the re-chemical formation may be performed at an
arbitrary stage an arbitrary number of times. It is preferable that
re-chemical formation be performed after the final formation step of the
semiconductor layer.

[0045]In the present invention, an electrode layer is provided on the
semiconductor layer. The electrode layer can be formed, for example, by
solidification of an electrically conducting paste, plating, metal
deposition or lamination of a heat-resistant electrically conducting
resin film. Preferred examples of the electrically conducting paste
include silver paste, copper paste, aluminum paste, carbon paste and
nickel paste. One of these may be used or two or more thereof may be
used. In the case of using two or more pastes, these pastes may be mixed
or stacked as separate layers. The electrically conducting paste applied
is then left standing in air or heated and thereby solidified.

[0046]Resin and electrically conducting powder such as metal are the main
component of the electrically conducting paste. If desirable, a solvent
for dissolving the resin or a curing agent for the resin is also used.
The solvent dissipates at the time of the above-described solidification
under heating. As for the resin, various known resins such as alkyd
resin, acryl resin, epoxy resin, phenol resin, imide resin, fluororesin,
ester resin, imideamide resin, amide resin and styrene resin are used. As
for the electrically conducting powder, a powder of silver, copper,
aluminum, gold, carbon, nickel or an alloy mainly comprising such a
metal, or a mixed powder thereof is used. The electrically conducting
powder is usually contained in an amount of 40 to 97 mass %. If the
electrically conducting powder content is less than 40 mass %, the
electric conductivity of the produced electrically conducting paste
disadvantageously becomes low, whereas if the content exceeds 97 mass %,
the electrically conducting paste may undergo adhesion failure and this
is not preferred. In the electrically conducting paste, the
above-described electrically conducting polymer for forming the
semiconductor layer or powder of metal oxide may be mixed and used.

[0047]Examples of the plating include nickel plating, copper plating,
silver plating, gold plating and aluminum plating. Examples of the metal
to be deposited include aluminum, nickel, copper, gold and silver.

[0048]More specifically, the electrode layer is formed by sequentially
stacking, for example, a carbon paste and a silver paste on the
semiconductor layer formed. By stacking layers up to the electrode layer
on the electric conductor in this way, a solid electrolytic capacitor
element is produced.

[0049]The solid electrolytic capacitor element of the present invention
having such a constitution is jacketed, for example, by resin mold, resin
case, metallic jacket case, resin dipping or laminate film, whereby a
solid electrolytic capacitor product for various uses can be completed.
Among these, a chip solid electrolytic capacitor jacketed by resin mold
is most preferred, in that reduction in the size and cost can be easily
achieved.

[0050]The resin mold jacketing is specifically described below. A part of
the electrode layer of the capacitor element obtained as above is laid on
one end part of a separately prepared lead frame having a pair of
oppositely disposed end parts, and a part of the electric conductor is
laid on the other end part of the lead frame. At this time, in a case
where the electric conductor has an anode lead, in order to adjust the
dimensions, the anode lead may be used after cutting off some end part
thereof. After connecting the above parts electrically or mechanically,
i.e., the former (one end part of the lead frame) is connected by
solidification of an electrically conducting paste and the latter (the
other end part of the lead frame) by welding, the entirety is sealed with
a resin while leaving a part of end of the lead frame outside the
sealing, and the lead frame is cut at predetermined portions outside the
resin sealing and bent (when the lead frame is present on the bottom
surface of resin sealing and the entirety is sealed while leaving outside
only the bottom surface or the bottom and side surfaces of the lead
frame, only cutting process without bending may be sufficient), whereby
the capacitor of the present invention is produced.

[0051]The lead frame is cut as described above and finally works out to an
external terminal of the capacitor. The shape thereof is a foil or flat
plate form and the material used therefor is iron, copper, aluminum or an
alloy mainly comprising such a metal. The lead frame may be partially or
entirely covered with at least one plating layer such as solder, tin,
titanium, gold, silver, nickel, palladium and copper.

[0052]After or before the above-described cutting and bending, the lead
frame may be subjected to various platings. It is also possible to plate
the lead frame before mounting and connecting the solid electrolytic
capacitor element thereon and again plate it at any time after sealing.

[0053]In the lead frame, a pair of oppositely disposed end parts is
present and a gap is provided between these end parts, whereby the
electric conductor part and the electrode layer part of each capacitor
element are insulated from each other.

[0054]With respect to the resin used for resin mold jacketing, a known
resin used for encapsulation of a capacitor, such as epoxy resin, phenol
resin, alkyd resin, ester resin and allyl ester resin, can be employed.
In all of these resins, when a low-stress resin (for example, a resin
containing usually 70 vol % or more of a filler and having a thermal
expansion coefficient α of 3×10-5/° C. or less)
generally available on the market is used, the encapsulation stress
imposed on the capacitor element, which is generated at the
encapsulation, can be mitigated and this is preferred. For the resin
sealing, a transfer machine is used with preference.

[0055]The thus-produced solid electrolytic capacitor may be subjected to
an aging treatment so as to repair the thermal and/or physical
deterioration of the dielectric layer, which is caused at the formation
of electrode layer or at the jacketing.

[0056]The aging treatment is performed by applying a predetermined voltage
(usually, within twice the rated voltage) to the capacitor. The optimal
values of aging time and temperature vary depending on the type and
capacitance of the capacitor and the rated voltage and therefore, these
are previously determined by performing an experiment. The aging time is
usually from several minutes to several days and the aging temperature is
usually 300° C. or less by taking account of thermal deterioration
of the voltage-applying jig.

[0057]The aging may be performed in any one condition of reduced pressure,
atmospheric pressure and applied pressure. Also, the aging atmosphere may
be air or a gas such as argon, nitrogen and helium, but preferred is
water vapor. When the aging is performed in water vapor and then
performed this in air or a gas such as argon, nitrogen and helium,
stabilization of the dielectric layer sometimes proceeds. The aging may
also be performed by supplying water vapor and then returning the aging
conditions to room temperature and atmospheric pressure, or may be
performed by supplying water vapor and then allowing the capacitor to
stand at a high temperature of 150 to 250° C. for several minutes
to several hours to remove excess water content. One example of the
method for supplying the water vapor is a method of supplying water vapor
from a water reservoir placed in the aging furnace by heat, or a method
of performing the aging in a constant temperature and humidity bath.

[0058]The method of applying a voltage can be designed to pass an
arbitrary current such as direct current, alternating current having an
arbitrary waveform, alternating current superposed on direct current, and
pulse current. It is also possible to once stop applying a voltage on the
way of aging and again apply a voltage. The aging may be performed while
raising a voltage from low voltage to high voltage in sequence.

[0059]The solid electrolytic capacitor produced by the method of the
present invention can be preferably used, for example, for a circuit
using a high-capacitance capacitor, such as central processing circuit
and power source circuit. These circuits can be used in various digital
devices such as a personal computer, server, camera, game machine, DVD
equipment, AV equipment and cellular phone, and electronic devices such
as various power sources. The solid electrolytic capacitor produced in
the present invention has a high capacitance and reliability and
therefore, electronic circuits or electronic devices obtained produced by
using the capacitor can give great satisfaction to users.

EXAMPLES

[0060]The present invention is described in greater detail below by
specifically referring to Examples, but the present invention is not
limited to these Examples.

Examples 1 to 6

[0061]A niobium primary powder (average particle diameter: 0.32 μm)
ground by utilizing hydrogen embrittlement of a niobium ingot was
granulated to obtain a niobium powder having an average particle diameter
of 120 μm (this niobium powder was fine powder and therefore,
naturally oxidized to contain 85,000 ppm of oxygen). The obtained niobium
powder was left standing in a nitrogen atmosphere at 400° C. and
further in argon at 700° C. to obtain a partially nitrided niobium
powder (CV: 286,000 μFV/g) having a nitrided amount of 7,500 ppm. The
resulting niobium powder was molded together with a niobium wire of 0.48
mm in diameter and the molded article was sintered at 1,270° C. to
prepare a plurality of sintered bodies (electrically conducting bodies)
having a size of 4.5×3.5×1.0 mm (mass of each sintered body:
0.07 g; the niobium lead wire was present such that 4.0 mm was inside the
sintered body and 10 mm was outside).

[0062]Thereafter, the sintered body was chemically formed in an
electrolytic solution containing a dopant described in examples 1 to 6 of
Table 1 at 80° C. with 20 V for 8 hours to form a dielectric layer
mainly comprising diniobium pentoxide on the sintered body surface and on
a part of the lead wire. After washing the sintered body with water and
dipping the sintered body in an alcohol solution, the sintered body was
dried to remove the alcohol. Subsequently, an operation of dipping the
sintered body in an 5% iron naphthalene-2-sulfonate alcohol solution,
drying it and then performing re-chemical formation in an aqueous
solution for chemical formation of each Example at 80° C. with 15
V for 5 minutes and drying it was repeated 5 times.

[0063]Furthermore, the sintered body was dipped in a bath (the bath was
laminated with a tantalum foil to serve as an external electrode)
containing a separately prepared mixed solution of 30 mass % ethylene
glycol and water, in which a trace amount of pyrrole monomer and 4%
anthraquinone-2-sulfonic acid were dissolved. By using the lead wire of
the sintered body as anode and an external electrode as the cathode to,
electrolytic polymerization was performed at 100 μA for 60 minutes.
The sintered body was pulled up from the bath, washed with water, washed
with an alcohol, dried and then subjected to re-chemical formation in an
electrolytic solution of each Example at 80° C. with 13 V for 15
minutes.

[0064]This operation of performing electrolytic polymerization and then
re-chemical formation was repeated 6 times, whereby a semiconductor layer
comprising polypyrrole was formed on the dielectric layer.

[0065]On this semiconductor layer, a carbon paste layer mainly comprising
water and graphite carbon was stacked and dried to be a carbon layer, and
then a silver paste mainly comprising 90 mass % of silver powder and 10
mass % of acrylic resin was stacked and dried to form an electrode layer.
In this way, 30 solid electrolytic capacitor elements were produced. The
conductors were placed such that the lead wire and the silver paste
surface of the electrode layer were in contact with end parts of a
separately prepared lead frame (copper alloy with both surfaces being
coated with 0.7-μm nickel base plating and 10-μm matte tin plating
further formed thereon) serving as an external terminal, and each was
electrically or mechanically connected by spot-welding for the former and
by a silver paste mainly comprising epoxy resin and silver powder for the
latter. Thereafter, the entirety excluding a part of the lead flame was
transfer-molded with epoxy resin and the lead frame outside the mold was
cut at a predetermined position and the remaining frame was bent along
the jacket to serve as an external terminal. In this way, chip solid
electrolytic capacitors each in a size of 7.3×4.3×1.8 mm were
prepared. Subsequently, the capacitors were aged at 125° C. with 7
V for 3 hours and then passed through a tunnel furnace in which the peak
temperature were 270° C. and the dwelling time in the region of
230° C. was 35 seconds. Subsequently, postplating were performed
on the capacitors in an electrolytic solution containing tin ion so that
the external terminals may recover from discoloration and further, the
capacitors were aged at 135° C. with 7 V for 3 hours to thereby
produce final chip solid electrolytic capacitors.

Comparative Example 1

[0066]Chip solid electrolytic capacitors were produced in the same manner
as in Example 1 except that the chemical formation was performed by using
dopant-free aqueous 1 mass % phosphoric acid solution instead of the
electrolytic solution used in Example 1 and that re-chemical formation
was performed in dopant-free aqueous 0.1% acetic acid solution.

Examples 7 to 12

[0067]Tantalum sintered bodies (each having a size of
4.5×1.0×3.0 mm and a mass of 83 mg, with a 0.40-mmΦ
tantalum outgoing lead wire 4.1 mm of which was present inside the
sintered body and 15 mm of which was present outside the body) each
having CV value (the value obtained by dividing a product of capacitance
and electrochemical voltage by mass value) of 150,000 μFV/g were used
as electric conductors. In order to protect the lead wires from splashing
up of solution at the later step for forming a semiconductor layer, a
tetrafluoroethylene-made washer was attached to each of the lead wires.

[0068]Each of the sintered bodies to serve as anode, excluding a part of
the lead wire, was immersed in an electrolytic solution containing each
dopant for Examples 7 to 12 as shown in Table 1. A current of 10 V was
applied between the anode and a tantalum plate electrode serving as
cathode to cause chemical formation at 30° C. for 7 hours to
thereby form a dielectric oxide film layer comprising Ta2O5.

[0069]The two operations of immersing each of these sintered bodies,
excluding the lead wire, in an aqueous 8% iron toluenesulfonate solution,
followed by drying at 100° C. and subsequently, performing
re-chemical formation at 30° C. with 9 V for 5 minutes in each
aqueous solution for chemical formation for each of Examples 7 to 12,
followed by drying, were repeated 5 times alternately.

[0070]Subsequently, the sintered body was immersed in a bath (which had
tantalum foil attached on its polypropylene-made bottom part to serve as
an external electrode) containing a mixed solution of 30 mass % ethylene
glycol and water, in which 3,4-ethylenedioxythiophene monomer and 4%
anthraquinone-2-sulfonic acid were dissolved each in an amount large
enough for insoluble portions to be present therein. By using the lead
wire of the sintered body as anode and the external electrode as cathode,
electrolytic polymerization was performed at 120 μA for 60 minutes.
The sintered bodies were pulled out from the bath, washed with water and
with alcohol, dried and then subjected to re-chemical formation in an
electrolytic solution for each Example at 30° C. with 7 V for 15
minutes. This operation of performing electrolytic polymerization and
then re-chemical formation was repeated 8 times, whereby a semiconductor
layer comprising a polythiophene derivative was formed on the dielectric
layer.

[0071]On this semiconductor layer, an electrode layer was formed in the
same manner as in Example 1 and the sintered bodies were sealed with
epoxy resin to produce chip solid electrolytic capacitors. Subsequently,
the capacitors were aged at 135° C. with 3 V for 6 hours and then
left standing in a furnace at 185° C. for 15 minutes to cure the
jacket resin, to thereby produce final chip solid electrolytic
capacitors.

Comparative Example 2

[0072]Chip solid electrolytic capacitors were produced in the same manner
as in Example 1 except that the chemical formation was performed by using
dopant-free aqueous 1 mass % phosphoric acid solution instead of the
electrolytic solution used in Example 7 and that re-chemical formation
was performed in dopant-free aqueous 0.1% acetic acid solution.

[0073]The performances of the capacitors produced in Examples 1 to 12 and
Comparative Examples 1 and 2 were measured by the following methods. The
results thereof are shown together in Table 2. The data in Table 2 show
each average value of 30 capacitors produced in each of the Examples.

Capacitance: The capacitance was measured at room temperature and 120 Hz
by using an LCR measuring meter manufactured by Hewlett Packard, Ltd.ESR:
The equivalent series resistance of the capacitor was measured at 100
kHz.Dielectric tangent: The value was measured at room temperature and
120 Hz by using an LCR measuring meter manufactured by Hewlett Packard,
Ltd.

High Heat Load Test:

[0074]Every ten of the capacitors produced in each Example were mounted on
one substrate by soldering (mounting condition: the capacitors were
passed three times through a reflow furnace in which the peak temperature
was 260° C. and the dwelling time in the temperature pattern of
230° C. or more was 30 seconds.). 2.5 V of voltage was applied
through wiring to each of the capacitors mounted on the total three
substrates for each of the Examples, the substrates were left standing in
a constant temperature bath at 105° c. for 2,000 hours and then
pulled out from the bath to room temperature.

[0075]As seen from comparison of Examples 1 to 12 with Comparative
Examples 1 and 2, when the dielectric layer is formed by performing
chemical formation in an electrolytic solution containing a dopant, the
produced solid electrolytic capacitor has high-capacitance with good
reliability.

[0076]Furthermore, as seen from comparisons of, such as Example 1 with
Example 3, Example 2 with Example 4, Example 7 with Example 9 and Example
8 with Example 10, quinonesulfonic acid is particularly excellent in
stability for a long period as contrasted with alkyl substituted benzene
(or naphthalene)sulfonic acid.